1. Field of the Invention
The present invention relates to an amplifier which is operated with a low power supply voltage and which has a reference voltage which temperature characteristic can be controlled.
2. Description of the Prior Art
A of prior art amplifier having a reference voltage independent of temperature has been conventionally arranged as disclosed in JP-A-Ho 2-193410. The amplifier comprises a transistor, a resistor and two of first and second current sources. A positively varying voltage to a temperature is obtained by passing a current through the resistor, connected at its one end to an input terminal and connected at the other end to the first current source which is connected in series with a negatively varying base/emitter voltage of the transistor to the temperature obtained by passing a collector current through the transistor from the second current source to cancel these positively and negatively varying voltages each other and to thereby obtain a reference voltage (about 1.25 V) independent of temperature, whereby there is obtained a comparison amplifier which acts as if an amplifier having one input connected to the reference voltage.
Since the output terminal voltage of each of the current sources are set to correspond nearly to the diode forward voltage, when such a band gap current source as shown in JP-A-60-191508 is employed, the power source voltage can be lowered down to about 0.9 V.
Thus, the comparison amplifier can be driven with the power source voltage lower than the reference voltage.
The above will be explained in more detail by referring to FIG. 15. FIG. 15 shows an arrangement of a prior art amplifier which has an input terminal 2 to which a voltage from a voltage source 1 is applied and also has an output terminal 3. In the drawing, reference numeral 51 denotes a resistor, numerals 52 and 54 current sources, 53 a transistor.
The operation of the prior art will next be explained. In FIG. 15, an addition of a base potential Vb53 of the transistor 53 to a multiplication of a resistive value R51 of the resistor 51 and a current Ics of the current source 52 corresponds to a voltage V1 of the voltage source 1 which is expressed by the following equation (1). EQU V1=Vb53+(R51.times.Ics) (1)
When the voltage Vl of the voltage source 1 is small, the base voltage Vb53 of transistor 53 becomes also small and the collector current Ic53 of transistor 53 becomes smaller than a current I54 of the current source 54. Thus, this causes a tendency of current to be discharged from the output terminal 3, so that the output voltage V3 becomes high. On the other hand, when the voltage V1 is large, the base voltage Vb53 of transistor 53 becomes also large and the collector current Ic53 of transistor 53 becomes larger than the current I54 of the current source 54. This causes a tendency of a current to be absorbed into the output terminal 3, so that the voltage V3 becomes low.
This operation is equivalent to the operation of the amplifier when an inverted input is connected to the input terminal 2, the reference voltage is connected to a non-inverted input, and an output is connected to the output terminal 3. The magnitude of the reference voltage can be found in the following manner. That is, when the voltage V1 of the input terminal 2 becomes equal to the reference voltage, no current flows into and out of the output terminal 3. When such a voltage V1 condition is found, the value of the reference voltage can be known.
First, since no current flows into and out of the output terminal 3, the following equation (2) is satisfied. EQU Ic53=I54 (2)
where, Ic53 denotes the collector current of the transistor 53 and I54 denotes the current of the current source 54.
At this time, the base potential Vb53 of the transistor 53 is expressed as follows. EQU Vb53=k.times.T/q.times.ln(I54/Is) (3)
where,
k: Boltzmann factor PA1 T: Absolute temperature PA1 q: Electric charge for an electron PA1 Is: The backward saturation current of the transistor
Meanwhile, the current source 52 is such a band gap current source as shown in JP-A-60-191508 and the current value Ics of the current source is determined by the following equation (4). EQU Ics=(k.times.T/q).times.ln(N)/Rcs (4)
where, N denotes a constant and Rcs denotes a current setting resistance.
Accordingly, the voltage V1 of the input terminal 2 under such a condition is expressed by the following equation 5) with use of the equations (1), (2) and (4) and the value V1' becomes the reference voltage of the prior art amplifier. EQU V1'=Vb53+(k.times.T/q).times.ln(N).times.R51/Rcs (5)
The first term in the equation 5) indicates the diode forward voltage and it is well known that the value of the diode forward voltage is about 650 mV and varies with temperature at a rate of -2 mV/deg.
Hence, when a change to temperature in the second term of the equation 5) is set to have such a value that is opposite in polarity to and is equal in magnitude to the first term, voltage changes to temperature in the first and second terms can be canceled each other. Thus, the reference voltage V1' can be eventually independent of temperature.
First, when a voltage change to temperature is found by differentiating the second term with respect to absolute temperature T and the differentiated voltage change is set to be equal to +2 mV the following equation (6) is obtained. ##EQU1##
Substituting the equation (6) into the second term of the equation 5) and setting T=300.degree. K. results in an equation (7) which follows. ##EQU2##
Hence, when the respective constants are set so that {(k.times.T/q).times.ln(N).times.R51/Rcs} or (R51.times.Ics) is 600 mV, the reference voltage V1' becomes about 1.25 V according to the equation 5) and thus can be eventually set to be independent of temperature.
Further, the base potential of the transistor 53 as the terminal voltage of the current source 52 corresponds to the diode forward voltage and the terminal voltage of the current source 5 is determined by a load connected to the output terminal 3. However, when the base of such a common-emitter transistor as the transistor 53 is connected to the output terminal 3, the base potential becomes the diode forward voltage. Thus, when the current sources are realized with such an arrangement as shown in JP-A-60-191508, the power source voltage can be lowered to about 0.9 V. Accordingly, the amplifier can be driven with a power source voltage lower than the reference voltage.
In this way, in the prior art amplifier, a reference voltage (about 1.25 V) independent of temperature can be obtained and the power source voltage of the amplifier can be lowered to about 0.9 V.
However, the prior art amplifier has had a first problem that the amplifier requires two current sources, which results in that the necessary circuit area becomes large.
A second problem in the prior art amplifier has been that the reference voltage is fixed at about 1.25 V so that, when it is desired to set a large reference voltage, this is realized by providing a resistor voltage-division means to the input terminal of the amplifier; whereas, when it is desired to set a small reference voltage, this is difficult because the value of the second term of the equation 5) must be made small while undesirably admitting its temperature dependency. That is, the reference voltage value and the temperature characteristic cannot be controlled independently of each other.